1. Field of the Invention
The present invention relates to an oscillation device, an optical deflector using the oscillation device, and an image display device and an image forming apparatus using the optical deflector, and a method of manufacturing the oscillation device.
2. Related Background Art
In recent years, an electromagnetic actuator has been tried to be formed on a substrate made of silicon or the like by utilizing a semiconductor process. If the electromagnetic actuator is formed by utilizing the semiconductor process, a stator, a moving element and an electromagnetic coil can be collectively formed, and hence a process for joining and bonding is unnecessary. As a result, the stator, the moving element and the electromagnetic coil can be aligned with one another with high accuracy. In addition, since those elements can be formed on a massive scale at a time, reduction in cost can be expected.
As one of examples of application of the electromagnetic actuator formed on a substrate, there is an optical deflector. An optical deflector is used in an image forming apparatus such as a laser beam printer, an image display device such as a head mounted display, or an image input device such as a bar code reader. There is seen even such a device in which a light can be deflected with respect to two axes with one optical deflector.
As an example in which an electromagnetic actuator formed on a substrate is applied to an optical deflector and deflection is possible with respect to two axes, there is known one disclosed in Japanese Patent Application Laid-Open No. 2000-235152. FIG. 10 is a plan view showing an optical deflector which is described as one of examples in Japanese Patent Application Laid-Open No. 2000-235152. This optical deflector is a torsion beam optical deflector, and is used as a deflector for two-dimensionally scanning an objective surface with a laser beam. This torsion beam optical deflector is constituted by an inner y-axis direction deflection portion 1003 and an outer x-axis direction deflection portion 1004. The inner y-axis direction deflection portion 1003 is constituted by a substrate 1001 having a groove portion 1002, a movable plate 1006 which is oscillatively supported by an axis portion 1005 and which has a thin film showing hard magnetism formed on its surface, a pair of thin film electromagnet portions 1007 for oscillating the movable plate 1006, and a mirror 1008 provided on the movable plate 1006. The surfaces having the movable plate 1006 and the thin film electromagnet portions 1007 formed thereon are slightly shifted in a direction of thickness. The movable plate 1006 is oscillated with the Coulomb force generated between a magnetic field generated by causing an AC current of 60 kHz as a structural resonance frequency of the y-axis direction deflection portion 1003 to flow through the thin film electromagnet portions 1007 and a magnetic field generated in the hard magnetism thin film formed in the movable plate 1006, and an applied light is deflected by the mirror 1008. It is possible to realize low power consumption because of the driving method utilizing the mechanical resonance. The outer x-axis direction deflection portion 1004 is the same in structure as the inner y-axis direction deflection portion 1003, and hence the driving method in the former is also the same as that in the latter. A driving frequency of this optical deflector is 60 kHz (in the y-axis), and 60 Hz (in the x-axis), and a deformation angle of the movable plate 1006 is ±13.67 degrees (in the y-axis direction).
In addition, as another example thereof, an electromagnetic actuator is tried to be miniaturized using the semiconductor process and permanent magnets, and the resulting product is applied to an optical deflector. In such cases, by using the permanent magnets, a magnetic field can be relatively readily formed, and also by lightening the moving element, high-speed operation can be expected. As one example thereof, there is known one disclosed in U.S. Pat. No. 5,606,447. FIG. 11 is a plan view showing an optical deflector which is described as one of examples in U.S. Pat. No. 5,606,447. In this optical deflector, a flat plate-like movable plate having a mirror is oscillatively supported with respect to a substrate by two torsion springs. In FIG. 11, reference numeral 801 designates a galvano-mirror; 802, a silicon substrate; 803, an upper side glass; 804, a lower side glass; 805, a movable plate; 806, a torsion spring; 807, a plane coil; 808, a total reflection mirror; 809, a contact pad; and 810A, 810B, 811A and 810C, permanent magnets. The driving plane coil 807 for generating a magnetic field by the current flow therethrough is provided in a peripheral portion of the movable plate 805, and the permanent magnets 810A, 810B; 811A, 810C are provided in pairs on the upper and lower surfaces of the semiconductor substrate through the upper and lower glass substrates 803 and 804 so that an electrostatic magnetic field is given only to both the sides of the driving plane coil parallel to the axial direction of the torsion spring 806. In this optical deflector, a current is caused to flow through the driving plane coil 807. Then, the Lorentz force F (not shown) acts on a direction in accordance with the Fleming's left hand rule on the basis of a direction of the current caused to flow through the plane coil 807 and a direction of the magnetic flux density provided by the permanent magnets 810A, 810B; 811A, 810C to thereby generate a moment adapted to oscillate the movable plate 805. At the time when the movable plate 805 has been oscillated, a spring reaction F′ (not shown) is generated due to spring rigidity of the torsion spring 806. If an AC current is caused to flow through the plane coil 807 to repeatedly operate the optical deflector, then the movable plate 805 having a light reflecting surface is oscillated to thereby scan an objective surface with the reflected light. In U.S. Pat. No. 5,606,447, there is also disclosed, as another example, an optical deflector in which the galvano-mirror 801 is installed instead of the total reflection mirror 808 to form a nesting structure to enable the two-axis scanning.
In addition, although not applied to an electromagnetic actuator, there is also an optical deflector which is aimed to realize the two-axis direction scanning with a simple structure using movable plates of a nesting structure. In this case, a shape of a movable plate having a deflection portion is parallelogram to thereby enable the two-axis direction scanning using a driving unit in one-axis rotating direction. As one example thereof, there is known an optical deflector disclosed in Japanese Patent Application Laid-Open No. 2001-75042. FIG. 12 is a plan view showing one of examples disclosed in Japanese Patent Application Laid-Open No. 2001-75042. In FIG. 12, reference numeral 901 designates an optical deflector; 902, a first torsion spring; 903, a second torsion spring; 904, a first movable plate; 905, a second movable plate; 906, a supporting substrate; 907a and 907b, electrodes; and 908, a fixed portion. The first movable plate 904 is supported to the supporting substrate 906 through the fixed portion 908 and the first torsion spring 902. The first movable plate 904 supports the second movable plate 905 through the second torsion spring 903 orthogonal to the first torsion spring 902. A shape of the second movable plate 905 is the parallelogram. The center of gravity G of the second movable plate 905 is located at an intersection between a first rotation axis A and a second rotation axis B. In addition, the supporting substrate 906 has the two electrodes 907a and 907b formed thereon, and thus a suitable voltage can be selectively applied across the electrodes 907a and 907b and a rear face of the first movable plate 904. A deflection portion such as a mirror is installed on the second movable plate 905 so that the resultant structure acts as an optical deflector. In a state of no application of a voltage, each of the first movable plate 904 and the second movable plate 905 is located at a neutral position. If a suitable voltage is alternately applied to the electrodes 907a and 907b, then an electrostatic force alternately acts on the left and right end portions of the first movable plate 904 to oscillate the first movable plate 904 with the first rotation axis A as a center. At the same time, a rotation moment acts on the second movable plate 905 to oscillate the second movable plate 905 with the second rotation axis B as a center due to asymmetry of its mass.
However, any of the above-mentioned optical deflectors has the following problems.
In the optical deflector disclosed in Japanese Patent Application Laid-Open No. 2000-235152 shown in FIG. 10, the high-speed operation round the two axes is realized. However, since the core constituting the thin film electromagnet portion 1007 is a thin film formed through the sputtering process, there is a limit to an increase of its cross section. For this reason, if a large current is caused to flow through the thin film electromagnet portion 1007, then a magnetic flux is inevitably saturated. Hence, it is difficult to further increase the deformation angle. In addition, the surfaces having the movable plate 1006 and the thin film electromagnet portion 1007 formed thereon are only slightly shifted in a direction of thickness, and hence from this viewpoint as well, it is difficult to further increase the deformation angle.
In the optical deflector disclosed in U.S. Pat. No. 5,606,447 shown in FIG. 11, if a deflection angle of a light during light scanning is intended to be increased, then it is necessary to increase a distance between the upper and lower glass substrates 803 and 804, and the movable plate 805. Thus, a relative distance between the permanent magnets 810A, 810B; 811A, 810C and the driving plane coil 807 becomes large accordingly, which results in reduction in the magnetic flux density in the plane coil 807, so that a large amount of current is required for the driving.
In the optical deflector described in Japanese Patent Application Laid-Open No. 2001-75042 shown in FIG. 12, the shape of the second movable plate 905 having the deflection portion is the parallelogram. Thus, if a light having a circular beam spot such as a laser beam is tried to be deflected, then the size of the second movable plate 905 needs to be increased more than the area which a light strikes. Thus, if the size of the second movable plate 905 is increased, then it is difficult to expect higher speed operation and more miniaturization.